block: switch bdgrab to use igrab

All of the current callers already have a reference, but to prepare for
additional users ensure bdgrab returns NULL if the block device is being
freed.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/block_dev.c

@@ -894,10 +894,14 @@ static struct block_device *bdget(dev_t dev)
 /**
  * bdgrab -- Grab a reference to an already referenced block device
  * @bdev:	Block device to grab a reference to.
+ *
+ * Returns the block_device with an additional reference when successful,
+ * or NULL if the inode is already beeing freed.
  */
 struct block_device *bdgrab(struct block_device *bdev)
 {
-	ihold(bdev->bd_inode);
+	if (!igrab(bdev->bd_inode))
+		return NULL;
 	return bdev;
 }
 EXPORT_SYMBOL(bdgrab);